One of the major problems to be solved in integrated optics lies in the area of waveguide bends. Waveguides must be able to corner in order to accommodate offsets in practical structures; the optimum packing of devices onto a substrate often requires the various devices to lie at differing angles relative to one another, and since such structures are typically long and thin it is necessary to provide large offsets or angular deviations over relatively short distances.
Waveguide structures vary in the degree to which they confine the light. In reasonably well-confined structures, such as titanium: lithium niobate guides, smooth curves have been used to reduce light power loss at bends, and proton exchange (PE) implants along the inner edges of the curves have been used to reduce bend radii considerably. The increased refractive index on the inside of the curves slows the wave down in these regions and re-orientates the phase front as the wave progresses around the bend. However, recent studies have shown that, for weakly-guided structures, in which transmission power losses are expected to be lower, simple S-shaped curves (composed of arc segments) can be as lossy as discrete bends. Recent research into weakly-guided structures, disclosed in "loss single-mode InP/InGaAsp waveguides grown by MOVPE", by P. W. A. McIlroy et al, Electron. Lett. 23 (13) pages 701-703 (1987), in which guaternary waveguides are formed on InP substrates, shows that structures which are tightly confined in the vertical direction (normal to the plane of the waveguide and of the substrate) with very weak lateral guiding have yielded the lowest scattering losses to date. The purpose of the present invention is to minimise power loss at bends in such waveguides, subject to the constraints of weak lateral guiding, and preferably without increasing the number of fabrication steps.